The present invention relates to apparatus for measuring weak magnetic fields, the apparatus having a multiplicity of superconducting connections between a SQUID array which is arranged on a substrate provided with contact pads, and a gradiometer array.
It is known to use superconducting quantum interference elements which are known by the designation "SQUID" (Superconducting Quantum Interference Device) for measuring weak variable magnetic fields, for instance, in a field intensity range below 10.sup.-10 T, and in particular, below 10.sup.-12 T. These elements are preferably used in medical technology and especially in magnetocardiography, where magnetic fields of the heart in the order of about 50 pT are measured as well as in magnetoencephalography, where magnetic fields of the brain in the order of about 0.1 pT are measured. The apparatus essentially comprises a detector coil, which, together with a coupling coil, forms a flux transformer which is arranged together with the SQUID within a Dewar vessel in liquid helium. Electronic circuitry is associated with the SQUID for determining and processing signals. Since the magnetic fields to be measured are up to six orders of magnitude smaller than external interference fields, appropriate shielding is required. Because of its greater sensitivity, a DC-SQUID (direct-current SQUID) can be used which contains two Josephson junctions. The detector coil, together with an associated compensation coil, forms a so-called gradiometer. With gradiometers of the zero.sup.th, first or higher order, the biomagnetic near field which is still non-uniform in the gradiometer range, can be determined selectively (Rev. Sci. Instrum. 53 (12)), December 1982, pages 1815 to 1845).
In order to obtain a three-dimensional field distribution, measurements must be performed successively in time at different locations of the region to be examined. The difficulty then, however, arises that the coherence of the field data is no longer assured over the required measuring time and in addition, clinically intolerable measuring times result. One has therfefore gone to measuring with devices which contain several parallel measuring channels (Physica 107B (1981), pages 29 and 30, North Holland Publishing Company).
In an embodiment of apparatus of the kind mentioned above with a magnetometer or gradiometer array which contains a larger number, for instance, at least 10 and preferably more than 30 gradiometer coils, a multiplicity of conductors must therefore also be connected to the associated coupling coil of a corresponding SQUID array.
In the known embodiment of a DC-SQUID, the SQUID together with its coupling coil can be designed as a thin-film transformer, in which the flat spiral-shaped turns of the coupling coil are arranged above the SQUID. With this design, good magnetic coupling is obtained (IEEE Transactions on Magnetics, vol. MAG-17, no. 1, January 1981, pages 400 to 403).